In recent years, liquid crystal display apparatuses have been used as various display apparatuses such as television monitors and personal-computer monitors.
The liquid crystal display apparatuses share such a common problem of an out-of-focus moving image that a boundary between parts of different display luminance is blurred in displaying a moving image. The problem of an out-of-focus moving image is attributed to a hold display carried out such that the previously written display content is maintained in a pixel being in its non-select period, and is peculiar to hold display apparatuses such as liquid crystal display apparatuses and organic EL display apparatuses. That is, display apparatuses, such as CRT (cathode-ray tube) display apparatuses and plasma display apparatuses, which carries out an impulse display (i.e., a display that is carried out only in a light-emitting period) are free from the problem of an out-of-focus moving image.
Examples of a method for preventing an out-of-focus moving image in a liquid crystal display apparatus include a technique (pseudo-impulse driving) of carrying out a display pseudo-similar to an impulse display by time-dividing one vertical period (one frame) into a plurality of sub-frames and by writing a signal in one pixel more than once. That is, an out-of-focus moving image is effectively prevented in a hold display apparatus by carrying out a low-luminance display (i.e., a display similar to a black display) at least in one of the sub-frames by means of time-division driving.
The reason why an effect of preventing an out-of-focus moving image is obtained by means of pseudo-impulse driving will be briefly described below with reference to FIGS. 12(a) and 12(b).
FIG. 12(a) is a diagram showing how a boundary between two regions of different display luminance moves at the time of hold driving. The vertical axis represents time; the horizontal axis represents location. Similarly, FIG. 12(b) is a diagram showing how a boundary between two regions of different display luminance moves at the time of pseudo-impulse driving. In FIG. 12(b), which shows pseudo-impulse driving, one frame is equally divided into two sub-frames at a ratio of 1:1.
In cases where the boundary moves in this way, the line of sight of the observer moves in accordance with the movement of the boundary. That is, in FIG. 12(a), the line of sight of the observer is represented by arrows 101 and 102. Moreover, a luminance distribution as seen by the observer in the vicinity of the boundary is obtained by time-integrating the display luminance in accordance with the movement of the line of sight. For this reason, in FIG. 12(a), a region located on the left side of the arrow 101 is perceived to be as luminous as a region located on the left side of the boundary, and a region located on the right side of the arrow 102 is perceived to be as luminous as a region located on the right side of the boundary. Meanwhile, a region located between the arrows 101 and 102 is perceived as if the luminance gradually increased. It is this portion that is recognized as image blur.
Similarly, in the case of pseudo-impulse driving shown in FIG. 12(b), according to the luminance distribution as seen by the observer in the vicinity of the boundary, image blur occurs in a region located between arrows 103 and 104. However, the slope is steeper than in the case of hold driving shown in FIG. 12(a). This shows that the image blur is reduced.
Further, in addition to the problem of an out-of-focus moving image, the liquid crystal display apparatuses shares such a common problem that a liquid crystal element has low response speed. Because of such a problem of response speed, in a liquid crystal display apparatus, a luminance response level attained after a change in input gradation may not reach a level attained when the input gradation is at rest.
A generally-known technique for remedying such low response speed is overshoot driving. The overshoot driving is such a driving method that a liquid crystal element is forcibly driven at a high speed by applying, to the liquid crystal element, a voltage slightly higher or lower than an applied voltage that gives a desired gradation level is applied to a liquid crystal element in accordance with an increase or a decrease in input gradation so that the liquid crystal element is forcibly driven at a high speed. Such overshoot driving is disclosed in Patent Documents 1 and 2.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 174186/1991 (Tokukaihei 3-174186; published on Jul. 29, 1991)
Patent Document 2: Japanese Patent No. 2776090 (Tokkyo 2776090; published on Mar. 26, 1993)